The present invention relates to a power supply switching circuit for a portable telephone set or an automobile portable telephone set.
In portable telephone sets and automobile portable telephone sets (hereinafter collectively referred to as a compact portable telephone set), a power capacity of a battery (second power supply) is designed to be small in order to reduce weight, and when the telephone set is not carried, the power supply is switched to a fixed power supply (first power supply) to extend the life of the built-in battery, which serves as the power supply. In the prior art, a relay is used for switching the power supply.
FIG. 3 shows a prior art power supply switching circuit. A symbol a denotes a compact portable telephone set whose power supply is to be switched, numeral 1 denotes a relay for switching the power supply, numeral 2 denotes a battery such as NiH or NiCd built in the compact portable telephone set, and numeral 3 denotes a fixed power supply or other power supply. The power from the battery 2 and the other power supply 3 are selectively supplied to a control unit 5 and an RF unit 6 of the compact portable telephone set through contacts of the relay 1. Relay 1 has an exciting coil 1' thereof driven by a control current from a switching control circuit 4.
However, in the prior art power supply switching which uses a relay, the volume occupied by the relay shares a large portion of the total volume of the compact portable telephone set and this has been a barrier in reducing the volume and weight of the compact portable telephone set.